1. Field of the Invention
This invention relates to an air-fuel ratio control system for an engine in which the fuel injection amount is subjected to feedback correction according to the deviation of the actual air-fuel ratio detected by an air-fuel ratio sensor from a target air-fuel ratio determined according to the engine operating condition.
2. Description of the Prior Art
There has been known an air-fuel ratio control system in which the air-fuel ratio of the air-fuel mixture actually introduced into the engine is detected by an air-fuel ratio sensor, the detected air-fuel ratio is compared with a target air-fuel ratio and the amount of fuel to be injected is feedback-corrected according to the deviation of the detected air-fuel ratio from the target air-fuel ratio in order to improve control accuracy of the air-fuel ratio. Further, there also has been known an air-fuel ratio control system in which a lean sensor which outputs a signal substantially in proportion to the exhaust gas oxygen concentration is used as the air-fuel ratio sensor and the air-fuel ratio is feedback-controlled even when the actual air-fuel ratio is leaner than the stoichiometric air-fuel ratio, thereby improving fuel economy. (See Japanese Unexamined Patent Publication No. 59(1984)-208141, for example.)
In such air-fuel ratio control systems, a base fuel injection amount for a given engine operating condition is first determined referring to a map in which the fuel injection amount or the width of the fuel injection pulse is related to the engine speed and the engine load, and the amount of fuel to be actually injected is determined by correcting the base fuel injection amount according to various conditions. In order to compare the output of the air-fuel ratio sensor with a target air-fuel ratio to obtain a feedback signal, a map in which reference values representing the target air-fuel ratios are related to engine operating conditions is used for determining the reference value according to a given engine operating condition and the output value of the air-fuel ratio sensor is compared with the reference value, and then the base fuel injection amount is corrected according to the deviation of the output value of the air-fuel ratio sensor from the reference value. Thus, the conventional air-fuel ratio control systems are disadvantageous in that various control maps of the type described above are needed and a large memory capacity is needed for storing the control maps. Further new control maps must be made when a new engine is developed, when the engine performance is changed, when the control characteristics are changed or when the injector is changed. Further, in the case of a map in which the width of the fuel injection pulse is related to the engine operating condition, widths of the fuel injection pulses optimal for engine operating conditions must be revised, thereby substantially adding to the manhours required for development.